1. Field of the Invention
The present invention relates to reverse osmosis process and equipment which are suitable for production of high-purity water for use in semiconductor and pharmaceutical industries, etc.
2. Related Art
After a pretreatment of industrial water or city water, such as coagulation and filtration, membrane filtration, activated carbon filtration, or decarbonation treatment thereof, a reverse osmosis treatment of the pretreated water has heretofore been effected using reverse osmosis equipment to produce high-purity water. A further treatment of permeate water resulting from permeation of the pretreated water through a reverse osmosis membrane in the reverse osmosis equipment has also heretofore been effected using ion exchange equipment, electrodeionization (EDI) deionized water production equipment, reverse osmosis equipment, or the like to produce higher-purity water.
Conventional reverse osmosis equipment is operated, or run, in such a state that the silica concentration of concentrate is equal to or lower than the solubility therein of silica. The reason for this is as follows. When the reverse osmosis equipment is run at a silica concentration of concentrate exceeding the solubility therein of silica, silica either alone or in chemical and/or physical combination with hardness ions [i.e., a hardness component(s)] such as calcium ions and/or magnesium ions, and/or other component(s) in the concentrate is precipitated along the flow path of the concentrate and on the surface of a reverse osmosis membrane to bring about a decrease in the flux of permeate water, an increase in pressure differential across the reverse osmosis membrane, etc. to thereby make the stable operation of the equipment difficult. Herein, the term "flux of permeate water" means the flow rate, per unit area of the reverse osmosis membrane, of permeate water flowed through the reverse osmosis membrane.
Additionally stated, the solubility of silica can be found, for example, using known data as shown on page 29 of "GYAKUSINTOHO-GENGAIROKAHO II OYO, MAKU RIYO GIJUTU HANDBOOD (REVERSE OSMOSIS-ULTRAFILTRATION II APPLICATION, HANDBOOK OF MEMBRANE UTILIZATION TECHNOLOGIES)" (published by Saiwai Shobo K. K. on Jun. 30, 1978). An example of such known data are shown in FIG. 1. Additionally stated, the values of the solubility of silica as shown in FIG. 1 are data on a system wherein silica alone is present in water. The values of the solubility of silica in the presence of hardness ions coexisting therewith are believed to be lower than those as shown in FIG. 1. Methods of lowering the silica concentration of concentrate to or below the solubility therein of silica include a method wherein the recovery of permeate water (amount of permeate water/amount of feed water) is controlled in such a way as to lower the silica concentration of concentrate to or below the solubility therein of silica. The silica concentration of industrial water as well as city water in Japan is around 20 ppm (as SiO.sub.2, the same will apply hereinbelow) in most cases, where the upper limit of the recovery of permeate water is 75 to 80% and concentrate in an amount corresponding to 20 to 25% of feed water is therefore discarded. This is problematic from the standpoint of effective utilization of feed water.
Alternatively, a reverse osmosis membrane having a low rejection of silica of, e.g., at most 50% may be used to run the reverse osmosis process while maintaining the silica concentration of concentrate at or below the solubility therein of silica. In this case, although the recovery of permeate water can be increased, the silica concentration of the permeate water turns out to be comparatively high, thus sacrificing a high degree of purification of the permeate water, attainment of which is the primary purpose of reverse osmosis equipment.
Accordingly, an object of the present invention is to provide reverse osmosis process and equipment which are stably operable without precipitation of silica even if the silica concentration of concentrate exceeds the solubility therein of silica (this solubility being one which has heretofore been believed to prevail, hereinafter referred to as "standard solubility"), and are therefore operable at a high recovery of permeate water.